I've been reading the books and scans, and I think they're both exhibitionists--they're always having sex where other people might see them! Katou usually initiates it, but Iwaki always goes along with it, so I think he's secretly into it, too! ;)
I love Katou being all possessive, and Mochimune seems like an interesting character. I've just reached the story arc where he gets introduced, so now I've got to go catch up on my reading!